1. Field of the Invention
The present invention generally relates to an ion conductor and an electrochromic display device, and more particularly to a new ion conductor in which ion mobility may be changed by an applied voltage, and an electrochromic display device using the ion conductor and having faster response speed of coloring and decoloring and excellent memory characteristics.
2. Description of the Related Art
Recently, extensive research has been made in the field of an electronic paper as an electronic medium which is replacing a paper medium. Although being a display device, the electronic paper is expected to be used just like ordinary paper. Therefore, among the requests for the electronic paper, there may be characteristics which are different from those for traditional displays such as a CRT (Cathode Ray Tube) and an LCD (Liquid Crystal Display). Namely, for example, the electronic paper may be requested to have characteristics as a reflective display device, to have a higher white reflectivity and a higher contrast ratio, to display high-resolution images, to be a display having a memory effect, to operate even with a low voltage input, to be thin and light weight, and to be inexpensive. Among those, there are strong demands for having the same white reflectivity and contrast ratio as that of paper as the characteristics directly related to the quality of the display.
As a display method used in the display devices for an electronic paper use, for example, there are a method of using a reflective liquid crystal, a method of using cataphoresis, and a method of using a toner migration. However, when any of the above method is used, it may still be very difficult to display multiple colors while maintaining a higher white reflectivity and a higher contrast ratio. Generally, to display multiple colors, a color filter may be used. The color filter, however, may absorb light, and therefore, the reflectivity may be reduced. Also, by using the color filter, one pixel is divided into three regions (i.e., red (R), green (G), and blue (B) regions). Therefore, the reflectivity of the display device may be reduced and the contrast ratio may be reduced accordingly. When the white reflectivity and the contrast ratio are greatly reduced, the visibility may also be degraded. As a result, it may become difficult to use the display device as such as an electronic paper.
On the other hand, there is one potential technique of providing the reflective display device using an electrochromic phenomenon without using a color filter. The electrochromic phenomenon is also called electrochromism in which when a voltages is applied, based on the polarity of the voltage, redox reactions occur, so that the color is reversibly changed. A display device using color development and color disappearance (hereinafter may be simplified as “coloring/decoloring”) of an electrochromic compound that induces the electrochromism (electrochromic) phenomenon is called an electrochromic display device. The electrochromic display device may be used an electronic paper display device because the electrochromic display device is a nonradiative display device, has a memory effect (i.e., memory characteristics to maintain a color and an image for a necessary time period), and operates with a lower voltage input.
On the other hand, the electrochromic display device may have the drawback of a slow response speed in coloring/decoloring (developing and disappearing a color) because the coloring/decoloring is performed based on redox reactions. For example, Japanese Patent No. 3955641 discloses a technique of improving the response speed of coloring/decoloring by fixing (disposing) the electrochromic compound near the electrode. However, the electrochromic is an electrochemical phenomenon. Therefore, it is the characteristics of an electrolyte layer (e.g., ion conductivity) that may largely influence the response speed or the memory effect of developed color. Further, when the electrolyte layer is in a liquid form where electrolyte is dissolved in a solvent, faster response speed (responsiveness) may be achieved. On the other hand, when the electrolyte layer is in a liquid form, the strength and the reliability of the device may be degraded. Therefore, to improve the strength and the reliability of the display device, researches have been made to solidify or gelatinize the electrolyte layer.
Namely, conventionally, in the battery and the electrochromic display device, an electrolyte solution is generally used. Therefore, a leakage of the electrolyte solution, and drying in the battery due to the volatilization of the solvent may occur. Further, in a typical battery case, the electrolyte solution may be unevenly distributed. Due to the uneven distribution of the electrolyte solution, a part of a separation membrane may dry. As a result, internal impedance may increase and an internal short-circuit may also occur. Especially, the electrochromic display device is used for a display purpose. Therefore, it may at least be necessary that one direction (side) of the electrochromic display device is made of a transparent material such as glass, plastic or the like. Therefore, in the electrochromic display device, it may be difficult to completely seal the electrolyte with a metal or the like. As a result, in the electrochromic display device, leakage of the electrolyte solution and drying in the battery due to volatilization of the solvent may become a serious problem.
To resolve the problem, a method of using a polymer solid electrolyte is proposed. More specifically, a solid solution made of matrix polymer and inorganic salt is proposed. In this case, the matrix polymer includes an oxyethylene chain or an oxypropylene chain. However, this solid solution is a perfect solid. Therefore, it may be easy to process the solid solution, but the electrical conductivity of the solid solution is three orders lower than that of a normal non-aqueous electrolyte solution. This lower electrical conductivity may be a (serious) problem in practical use.
Further, to improve (reduce) the volatility, there are proposals of including a liquid crystal material into the electrolyte layer. In one example, a polymer liquid crystal material is included into the electrolyte layer. By doing this, the volatility of the electrolyte layer may be improved (reduced), and the adhesiveness to the electrochromic layer may be improved (e.g., Japanese Laid-open Patent Publication No. 2002-287172). In another example, an ion conductor is disclosed in which the ion conductivity between the electrodes (in the vertical direction) is improved by including (combining) the electrolyte and a specific low-molecular nematic liquid crystal material (e.g., Japanese Laid-open Patent Publication No. 2007-194150).
However, the electrolyte layers in related art may not be sufficient for the electrochromic display use. Namely, the drive response may be improved due to the improved ion conductivity of the low-volatile electrolyte layer. On the other hand, condurrently, the memory effect of the image display is more likely to be degraded. This may occur because, generally, the electrochromic reaction is not bistable.
The display pixels of the electrochromic display device have characteristics similar to those of a capacitor in that a charge is stored. Further, when a voltage is applied to the pixel, electricity is conducted to a pixel (i.e., the pixel is turned ON) so that the color is developed while the electricity is charged. After the electricity is charged, an open-circuit configuration is formed so as to maintain the colored state. This is what is called memory effect (hereinafter may be referred to as “memory characteristics”). In this open-circuit configuration, however, when ions diffusively move in the electrolyte layer, the oxidation state or the reduction state may become unstable. As a result, it may become difficult to maintain the charged colored state. Especially, when the ion conductivity of the electrolyte layer is relatively high (e.g., when the electrolyte layer is a liquid state), ions are more likely to diffusively move. As a result, the display image may blur.
As a display device, from a viewpoint of the display resolution and the display speed, it is preferable that the display device has a configuration so that a color is independently developed and disappeared (hereinafter may be referred to as “colored and discolored”) in each of the pixel electrodes which are formed in a matrix manner. However, when this configuration is used in, for example, a high-resolution display, the blur in the display image may become remarkable. Further, as an element configuration, to form a drive circuit for a TFT (Thin Film Transistor), it is preferable that an element electrode having a lower light transmission rate is disposed on the opposite rear side (as a counter electrode opposing to the display electrode) and a sheet electrode having a higher light transmission rate such as ITO (Indium Thin Oxide) is disposed as a display electrode from a viewpoint of the light use efficiency and color quality. However, when this configuration is used, the blurring speed of the display image may be increased. This is because it is thought that electric charges are more likely to move in the sheet electrode direction due to the higher light transmission rate.
To resolve the problem, Japanese Laid-open Patent Publication No. 2008-304906 proposes a technique in which the electrolyte layer is formed only on a part corresponding to the display image to prevent the diffusion, so that only selected pixels are displayed. However, this configuration may be too complicated, so that it may be difficult to manufacture this configuration, and the cost may be greatly increased.
On the other hand, in Japanese Laid-open Patent Publication No. 2010-33016, the inventors of the present invention propose the electrochromic display device having a configuration in which a plurality of display electrodes and the corresponding electrochromic layers are accumulated on a display substrate. In this electrochromic display device, it may become possible that a plurality of colors are independently developed using a simple method, so as to display a color image. Further, in this electrochromic display device (of Japanese Laid-open Patent Publication No. 2010-33016), as the electrolyte of the display device, perchlorate chloride and the like is used, and no electrolyte in a liquid crystal state is used.
As described above, in the electrochromic display device in related art, it may still be difficult to achieve sufficient response speed in coloring and decoloring a color, sufficient memory characteristics, and sufficient reliability of the devices.